Currently, many gas turbine engines operating at power plants utilize steam injection systems as a low-cost form of power augmentation. Typically steam injection is considered a relatively constant process because the start-up time and shut-down time of the steam injection system is typically on the order of thirty to sixty minutes.
Some steam injection systems of the prior art have a steam bleed near the injection point so that the steam can be used to heat the steam injection pipes. As steam injection is initiated through cold pipes, the hot steam condenses and forms water which is vented through the steam vent. Steam flow is gradually increased until the pipes are sufficiently heated, at which point water no longer forms. The steam vent then closes and the system is ready to inject steam into the gas turbine engine. This heating process for the steam pipes typically takes approximately thirty minutes.
Other prior art steam injection systems heat the steam injection pipes by slowly adding steam to the steam injection pipe and any resulting water that forms is injected into the gas turbine. Since the flow rates are extremely low, as water is typically not desired to be injected through a steam injection system into the gas turbine, this process typically takes in excess of thirty minutes to complete.
Steam is typically generated at a gas turbine site to either drive a steam turbine for producing additional power or is delivered to a supplemental process, such as an adjacent manufacturing facility, or in some cases both. When steam is generated for a supplemental process, the gas turbine power plant is referred to as a cogeneration plant, or cogen plant, because it is producing two products, electricity and steam. The recipient of the steam is called the steam host. Examples of a steam host can include a manufacturing or processing plant. Most cogen plants supply both electricity and steam to the steam host and sometimes the steam requirement and power requirement are not optimally balanced, so the cogen plant has to continuously optimize and balance the steam production and gas turbine output to try to meet the steam host's demand. This is a significant challenge, and as a result, there are periods when excess steam is produced as a result of the power requirement that cannot be avoided and in these cases, the cogen process loses efficiency.
Steam injection power augmentation systems are not typically deployed to meet short term spot market demands because they can take too long to come online and be available. For example, if there is a spike in power demand that is not expected to last long, then the steam injection power augmentation system is not advantageous to use. Additionally, steam injection systems are not considered optimal for fast-acting regulation devices due to their slow start-up speed.
A gas turbine incorporating a steam injection system in accordance with the prior art is depicted in FIG. 1. The gas turbine comprises a compressor 10 which compresses ambient air 20 to an elevated pressure and temperature and then discharges hot pressurized air into a compressor discharge case 14, or CDC. The compressor discharge case 14 is sometimes referred to as a wrapper because it houses the combustion and transition section of the gas turbine. The hot pressurized air enters the combustion chamber 12 where fuel 24 is added. The mixture of fuel and air is ignited and forms combustion gases. The hot combustion gasses are directed to the turbine section 16 which produces about twice the power being consumed by the compressor and therefore, the net power is delivered to a generator 18 for the gas turbine. As the hot gasses 22 exit the turbine section 16, the hot gasses 22 are directed into a heat recovery steam generator (HRSG) 605, where pressurized water is turned into pressurized steam 603 which exits the HRSG 605 and is directed into a steam turbine, a steam process, or both (602). When power augmentation with steam injection is desired, the steam injection isolation valve 113 is opened, the steam injection valve 114 is partially opened and the steam vent valve 616 is opened to allow a small amount of steam to flow through the steam injection piping 601. Water is typically formed as the steam system is warmed up and the water is drained as required through the vent valve 616. When the steam pipes 601 are heated and condensation no longer forms, steam 615 flows out of the vent system, the steam vent valve 616 is closed and the steam injection valve 114 is opened to allow the desired amount of steam injection into the gas turbine.
Typically the distance between the steam injection valve 114 and the steam injection isolation valve 113 can be several hundred feet. As a result, it takes about thirty minutes for the steam injection system to preheat to a desired operating temperature. Some steam injection systems do not have the steam vent valve 616 and therefore they can take even longer to preheat because of the sensitivity to injecting water into the gas turbine as the steam injection system is started up.